Acceptance of Game Center Challenges

I have been successful at issuing score challenges in Game Center by using the

challengeComposeController

method of

GKScore

. This

GKScore

object has a

context

property which holds a specific seed to be used to start a specific game when the challenged player accepts the challenge. My question is, when the challenged player presses the Play Now button in the Challenges screen of the Game Center View Controller, how can the game view controller know that the player accepted the challenge and which challenge he/she accepted when the

GKGameCenterViewController

is dismissed? I know there is a

GKLocalPlayerListener

protocol with different methods to manage challenges but it isn't very well documented when these methods fire or should be used.